Crest students work with neighbors to create demonstration bioswale pond

Peter Tarnawa, Jeff Garrett, Brian Rock and Bill Salsbury at the work site. - Contributed photo
Peter Tarnawa, Jeff Garrett, Brian Rock and Bill Salsbury at the work site.
— image credit: Contributed photo

What do the Crest Horticulture Program, the Ellis Pond neighborhood and the Mercer Island Right of Way and Storm Water manager have in common? They are all working to create a demonstration bioswale on Mercer Island.

But take note: a bioswale is not a large marine mammal.

Kahley Blankenship, a Mercer Island High School senior, who is enrolled in the Crest Horticulture Program, explains that “a bioswale is a natural system allowing water to flow so that impurities and chemicals are removed before the water flows into the lake.”

Rather than having a V-shaped ditch that allows water laden with pollutants to move quickly into the lake, ditches are re-shaped to have a wider bottom and are planted with native plants that filter the water into the soil. The water-soluble pesticides and road run-off, which contains motor oil and asbestos from brake linings, can easily flow into our watershed if they are not stopped. A bioswale provides a stopping place for this water, and plants’ roots direct this water to the bacteria in the soil, where carbon-based pollutants like motor oil can be broken down. Bioswales also reduce the amount of storm water that goes into the sanitary sewer system, thus reducing or preventing sanitary sewer discharge directly into bodies of water.

Carley Tallman’s Horticulture class at Crest has studied bioswales for several weeks, visiting demonstration projects in Seattle and evaluating the site on 89th Street next to Ellis Pond. Blankenship suggests using native plantings so that they may survive without much maintenance, once they are established.

“Bioswales beautify a neighborhood and can increase property values because they are better looking than a ditch,” said Blankenship, after visiting some before and after neighborhoods in North Seattle with her class.

When Bill Sansbury, Mercer Island Right of Way and Storm Water manager, was contacted about creating a bioswale demonstration project next to Ellis Pond, he was very enthusiastic. Glenn Boettcher, Mercer Island maintenance director, calls Sansbury “our point person on compliance with new federal storm water regulations.” To comply with federal and state permits, Mercer Island is required to develop and implement an operations and maintenance program with the ultimate goal of preventing or reducing pollutant runoff from the storm water system. For Mercer Island to be allowed to continue to discharge storm water into Lake Washington, it must follow the guidelines in these permits.

Another requirement of the permits involves a public education and outreach program. When the Ellis Pond neighborhood held its semi-annual park maintenance and planting party earlier this month, Sansbury and Brian Rock, generalist for the Mercer Island Maintenance Department, were on hand to reshape the V-shaped ditch into a bioswale-friendly water holding area. A rock check dam was installed to hold the water, allowing the plants to filter the water into the soil. Neighbors planted native plants provided by the city and donated by neighbors. Sansbury spoke at the Ellis Pond lunch gathering about the importance of bioswales and how the city is updating its methods for dealing with storm water runoff.

Homeowners can also do their part. The rain that falls on roofs, driveways and compacted soils collects quickly and can overwhelm ditches and pipes, causing flooded homes, eroded hillsides and sewer overflows. Homeowners who direct downspout runoff to a cistern or rain garden can reduce the amount of storm water flowing from their property and take the pressure off the public storm water systems. Rain gardens are a beautiful addition to a landscape, and there is an additional benefit of contributing to cleaner streams and lakes. Seattle Public Utilities has helpful information at its Web site:

One of the Ellis Pond neighbors who has been most active in promoting the idea of a demonstration bioswale is Peter Tarnawa. After visiting the North Seattle bioswale project, he states, “What impressed me with the bioswales in Seattle was the scale of the projects there.

“There are lots of things we can do on Mercer Island. Because all of our runoff goes into Lake Washington, we need to be very responsible and make sure our discharge is as clean as possible,” said Tarnawa. “Seattle is seeking out neighborhoods that support the concept of bioswales, and the city will provide utility rebates for homeowners who disconnect their downspouts and put the water into a qualified rain garden or cistern. I hope we will have similar programs on Mercer Island soon.”

For information, contact Marcia Mellinger at

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